It looks as though the retention system on those things will hold it no matter what direction your mags are pointing. The problem I see is that if you have to go to ground for even a second, especially in an arid environment, all sorts of pebbles, moon dust, cookie crumbs, whatever, will jam that locking mechanism up and allow your mags to fall out. An even worse scenario would be that they wouldn't allow you to access your mag at all due to a foreign object being jammed in there.
Pretty cool paintball set up though
